1. Field of the Invention
This invention relates to a process for treating metal particles deposited on a substrate with an organic solvent. In particular, it relates to a process for forming a two-dimensional ordered structure of metal particles deposited on a self-aligning membrane.
2. Description of the Prior Art
A single-electron device controls electrons to a single electron level utilizing a xe2x80x9ccoulomb blockadexe2x80x9d phenomenon where coulomb energy inhibits tunneling of electrons. A single-electron device has the following excellent properties.
(a) A temperature at which the device can operate increases as its size is reduced.
(b) A conductor or semiconductor material can be used.
(c) Since it can control a single electron, a power consumption is also reduced to one several ten thousandth compared to that for a conventional device.
In order to operate such a single-electron device at a room temperature, its capacitance must be a level of 10xe2x88x9218 F., which indicates a device size of several nanometers. There have been various attempts to construct an ordered structure with such a size, but no satisfactory processes have not been discovered.
The following two types of processes have been mainly studied for constructing an ordered structure in a nanometer level.
(i) Processes employing conventional lithography; and
(ii) Processes forming a fine structure in a self-aligning manner.
The lithography technique described in (i) has been developed to a level that a pattern with a size of about {fraction (1/10)}xcexcm can be processed. It, however, is not adequate for processing with good reproductivity in a several nm level. Drawing with a convergent electron beam has been mainly investigated. However, drawing with an electron beam, in which each pattern must be drawn with an electron beam, has drawbacks, e.g., that a throughput may be reduced and that an ordered structure formed may be damaged during electron-beam drawing.
Much attention has been paid to the process in (ii) above forming an ordered structure in a self-aligning manner, which depends on advance in study for metal particles with a size range of several nm to several ten nm and their agglomerates (clusters) in the fields of organic chemistry and catalyst chemistry.
An ordered structure used for a single-electron device may be made of a conductor in a nanometer level. Thus, this process applies metal particle agglomerates with a size of several nm to a single-electron device. A particularly noticeable process is that metal particles are coated with thiol molecules to form metal thiol particles.
Metal particles with a size of several nm are highly reactive because of a larger ratio of a surface area to a volume so that they may agglomerate when being left as they are. Metal particle surfaces are, therefore, coated with thiol molecules to prevent the metal particle from agglomerating.
Metal particles may be deposited on a substrate by first dropping a solution of metal thiol particles and then drying it. This process, however, has a drawback that the number of metal particles constituting an agglomerate (cluster) and pattern forming of an agglomerate (cluster) cannot be controlled.
A further advanced process is, for example, that metal particles are deposited on an SAM made of organic molecules; and the metal particles are treated with thiol and then dispersed on the SAM to form an ordered structure as described in T. Sato, D. Brown, F. G. Johnson, Chem. Commun., 1007 (1997).
The process will be described with reference to FIG. 3, in which a metal employed is gold.
On a silicon oxide film is formed an SAM using APTS (3-(2-amino-ethylamino)propyltrimethoxysilane). The substrate is immersed in a solution containing gold particle colloid (gold colloid solution) separately prepared to deposit the gold colloid on the SAM. Then, a loose electrostatic bond is formed between the amino group in APTS and the surface of the gold colloid so that the gold colloid is fixed on the SAM (FIG. 3(c)).
The term xe2x80x9cgold colloidxe2x80x9d as used herein refers to each gold colloid particle.
Gold colloids have a positive charge and thus repulse each other, so that they exist apart from each other by a distance depending on a charge on the colloids, without forming an agglomerate (cluster). At this stage, gold colloids cannot be bound each other to form an agglomerate (cluster). Furthermore, the maximum number for gold colloid per a unit area of a substrate on which they are loaded is limited (FIG. 3(a)).
After the treatment, the SAM on which gold colloids are loaded is immersed, together with the substrate, in a thiol solution to coat the metal particle surface with the thiol molecules.
A sulfur atom in a thiol molecule is very apt to be bound to gold. Specifically, a thiol molecule cleaves a loose electrostatic bond between a gold colloid and the amino group in the SAM to form a covalent bond with gold. Thus, gold particles are coated with thiol molecules to provide gold thiol particles (FIG. 3(d)).
Once coated with thiol molecules, gold thiol particles can be dispersed on the SAM surface. Thus, the gold thiol particles are dispersed while being bound each other via van der Waals force to form a two-dimensional ordered structure when being in contact with each other (FIG. 3(b)).
According to this conventional process, silver or platinum can be used as a metal to provide a two-dimensional ordered structure on an SAM.
As described above, this process is adequately sophisticated to provide a metal particle ordered structure with a size of several nm with good reproductivity.
A cycle of metal particle adhesion and thiol treatment may repeated to increase the number of metal particles on the SAM surface for providing a two-dimensional ordered structure with an extent comparable to the substrate area.
The process of the prior art has a drawback that metal particles loaded on the SAM are detached from the SAM during thiol treatment. Although it depends on some factors such as treatment conditions and the size of the metal particles, all metal particles loaded may be sometimes detached. It is because a xe2x80x9cbond between a thiol molecule and a metal particlexe2x80x9d is formed during thiol treatment so that an electrostatic bond between a metal colloid and an amino group in the SAM is cleaved. Then, metal particles which have been bound to the SAM surface are liberated so that they can be dispersed on the SAM. At the same time, an electrostatic bond is, however, lost so that a force binding metal thiol particles on the SAM becomes weaker and the metal thiol particles begin to be liberated in the solution.
Using gold as a metal, liberation of gold thiol particles from an SAM was significant when the size of the metal particles was less than 10 nm. Furthermore, when the size of the metal particles was 1 nm or less, metal particles little remain on a substrate. It was, therefore, very difficult to form an ordered structure.
FIGS. 3(a) and (b) schematically show such states. It is assumed that 107 metal particles exist on an SAM before thiol treatment. After thiol treatment, an ordered structure in which the metal thiol particles agglomerate is formed on the SAM while about a half of the metal thiol particles are detached from the SAM, so that only 63 particles remain.
In view of these problems, an objective of this invention is to prevent metal thiol particles from being detached from an SAM during coating metal particles deposited on the SAM with thiol molecules.
Another objective of this invention is to effectively form a metal particle ordered structure on an SAM without losing metal particles. A further objective of this invention is to provide a process for forming a metal particle ordered structure in which the size of the ordered structure of metal particles is adjustable.
This invention proposes a process for forming a metal particle ordered structure on a substrate, comprising the steps of:
(1) immersing a metal oxide film substrate in a solution containing at least APTS (3-(2-amino-ethylamino)propyltrimethoxysilane) to form a self-aligning membrane (SAM) of APTS on the substrate surface;
(2) immersing the substrate with the SAM in a solution containing metal particle colloid with a particle size (D) of 0.8xe2x89xa6Dxe2x89xa610 nm to load metal colloids on the SAM surface;
(3) immersing the substrate on whose SAM surface metal colloids are loaded, in a solution containing at least a material having a thiol group to thiolate the metal particles for providing metal thiol particles; and
(4) applying a given voltage between the substrate where metal colloids are loaded on the SAM surface as the first electrode and the second electrode in the solution while conducting the above step (3).
According to this invention, when treated metal colloids pre-loaded on an SAM with a thiol, a voltage can be applied between a substrate and a thiol solution to prevent metal thiol particles from being liberated from the SAM by the action of electrostatic attraction between the metal thiol particles and the substrate.
According to the prior art, as the size of metal particles decreases, after the thiol treatment the amount of the particles liberated from the SAM increases. It has been, therefore, very difficult to keep metal particles with a small size on the SAM. When using, for example, gold as a metal, gold particles with a size of 1 nm or less cannot be practically kept on an SAM.
According to this invention, however, even gold particles with a size of 1 nm or less it becomes possible to be easily kept gold thiol particles on an SAM.
In this invention, metal particles liberated from an SAM are less than the prior art. This invention can, therefore, eliminate loss of metal particles to effectively form a metal particle ordered structure.
A voltage applied between the substrate and the thiol solution may be varied to adjust an effective moving length of the metal thiol particles on an SAM and thus to easily control the size of the metal particle two-dimensional ordered structure. The term xe2x80x9ceffective moving lengthxe2x80x9d of metal thiol particles as used herein means a moving distance per a unit time of the metal thiol particles on an SAM, indicating diffusibility of the metal thiol particles on the SAM. A longer effective moving length indicates that metal thiol particles can be more easily diffused on an SAM.
Specifically, a lower applied voltage during thiol treatment allows individual metal thiol particles to more easily move on an SAM, i.e., a longer effective moving length. A probability of collision between metal thiol particles is, therefore, increased. In this case, a large two-dimensional ordered structure may be easily formed even with a small number of particles.
On the other hand, a higher applied voltage restricts movement of individual metal thiol particles on an SAM, i.e., a shorter effective moving length. A probability of collision between metal particles on the substrate is, therefore, reduced so that a small two-dimensional ordered structure may be formed even with a large number of particles.
An effective moving length depends on the size of metal particles. If the other conditions are identical, a larger particle size gives a smaller effective moving length of metal thiol particles.
As described above, this invention allows the size of a two-dimensional ordered structure and its surface density to be controlled via the size of metal particles and an applied voltage.